Published Unexamined Japanese patent application (PUPA) No. 59-154630 (154630/84) discloses that an optical recording medium on which recorded clock pulse signals are recorded is fixed in a freely replaceable manner, on a rotary shaft to which a magnetic recording medium is fixed, and the surface of the recording medium is read by a combination of light source and optical sensor to generate clock pulses.
In accordance with this method, signals following the rotational characteristics, that is wow and flutter components, of a motor for driving the rotary shaft can be obtained.
However, if an error occurs in the optical sensor or a recording or mounting error occurs in the optical recording medium when the above-mentioned prior art is applied, an erroneous signal might be issued from the optical sensor It is preferable not to generate sector boundary signals for servo write operations based on such signals.